Sand migration is a constant problem when fluids are produced from unconsolidated subterranean formations. When loose sand is removed from the formation by fluids flowing toward a wellbore, the sand can plug fluid passages, damage oil production equipment, and settle from the produced fluids as a sludge. The sludge creates a maintenance and disposal problem. Sand migration is minimized by placement of sand or fine gravel around a screen or slotted liner in the wellbore with the fluid producing formation. This is typically referred to as a gravel pack. The gravel pack provides a volume for the migrating sands to deposit or be filtered from the production fluids.
Gravel packs are often most effective in an open hole completion. An open hole completion does not utilize a casing within the producing formation. A gravel pack may be placed between a perforated casing and a smaller diameter slotted or screened liner, but the produced fluids must be channeled through the perforations, where fluid velocities are increased. The gravel packing is less effective in the regions of higher fluid velocities. Open hole completions are therefore generally preferred when a gravel pack is necessary.
Multiple zone completions are also often desirable. A multiple zone completion utilizes a single wellbore to produce fluids from mulitple strata. The oil producing strata may be separated by brine bearing strata, in which case the brine bearing strata must be cased and cemented. Multiple zone completions are often not practical due to differeing formation pressures, and therefore the possibility of cross flow between strata. But when multiple zones can be produced from each wellbore, the cost savings are obviously significant.
When multiple zone completions can be utilized, open hole completions have generally not been utilized in the upper zones. To utilize open hole completions in a well which is to produce from multiple zones, it has been necessary to case the entire well, cement the well, and then mill out casing and cement within the strata to be produced. The zones between the producing zones must be cased in order to accomplish a good cement job between producing formations. Milling of casing to permit open hole completions in upper zones is expensive and time consuming. It can take about one hour of milling for each foot of casing to be removed. A drilling rig is therefore required for about ten extra hours for each ten-foot-thick zone to be completed. Although expensive, this process has occasionally been utilized because of the accumulative benefits of multiple zone completions and open hole completions.
Fiberglass casing is also known in the art. Fiberglass casing is particularly useful when fluids or gases being injected or produced through the wellbore are highly corrosive, but the use of fiberglass is limited to wells in relative low temperature formations due to plastic deformation temperatures which approaches the temperatures of many production wells. Fiberglass casing is also only useful at fairly shallow depths due to limited collapse strength. Further, fiberglass casing is generally more expensive then steel casings.
It is therefore an object of the present invention to provide a method to provide a multiple zone open hole completion wherein the completion process is considerably less time consuming and thus less expensive than the prior art processes of milling out cemented steel casings within the producing formations. In another aspect, it is an object to provide a casing string which comprises a combination of readily millable segments and steel sections wherein the readily millable segments align with producing strata which intersect a wellbore.